Sudoku Locked Candidates Method

The "Locked Candidates" method is a more advanced method that can be used when the Single Solution and Single Cell methods cannot solve the puzzle on their own. This may be required for medium difficulty puzzles. This method is also known as the Single Box or Block-Block method.

In the first example on the left, look at the center sub-square. Notice how the number 2 can only appear in the top row. Since it must appear in the top row of the center sub-square, we know that it cannot appear in the top row of the adjacent sub-square. Thus we can eliminate the number 2 as a possibility from the cells marked with yellow.

In the second example, look at the fourth row. Notice how the number 2 must occur in the top row of the middle right sub-square. Since it must occur in one of these two spots, we can eliminate 2 as a possibility in the cells marked with yellow.